1. Field of the Invention
The present invention generally relates to a photoelectric conversion device for converting a photo signal into an electric signal, and particularly relates to a photoelectric conversion device which uses an amorphous semiconductor having a charge multiplication action so as to realize high sensitivity. The photoelectric conversion device according to the present invention includes an image pickup device of the photoconductive film lamination type, for example, a one-dimensional image sensor, a two-dimensional image sensor, etc.
2. Description of the Related Art
Conventionally, as the photoelectric conversion device mainly constituted by an amorphous semiconductor, known are such a photocell and a one-dimensional image sensor as disclosed in JP-A-52-144992, and such a two-dimensional image sensor (hereinafter, referred to as "solid-state photoconductive device of the photoconductive film lamination type") which is constituted by a combination of a solid state drive circuit and an amorphous photoconductive material as disclosed in JP-A-49-24619.
As photoelectric conversion portions of these photoelectric conversion devices, there are known two types: one which itself has no multiplication function and the other which itself has a multiplication function.
With regard to the available photocells, one-dimensional image sensors and solid-state imagers of the photoconductive film lamination type, the most of them belong to the former type i.e., the one which itself has no multiplication function, and the photoelectric conversion portion of the former type is further classified into two types depending upon whether charge injection from the electrode thereof is present or not. A first one is of a so-called blocking type in which the charge injection from the electrode thereof into the photoconductive film is blocked and a second one is of an injection type in which such charge injection is allowed. Of these two types, the blocking type devices are generally utilized for a case wherein a good photo-response is required. In this case, however, it is impossible to derive signal charges greater than the number of carriers produced by the incident light. That is, the gain of photoelectric conversion is 1 at best. In the injection type device, on the other hand, a high sensitization of a gain larger than 1 can be realized because, in principle, charges more than the number of the incident photons can be derived into an external circuit. In this system in which charges are partly injected into the inside of a sensor, however, the photo-response is considerably deteriorated.
With regard to the photoelectric conversion devices of the type in which a photoelectric conversion portion thereof has a multiplication function, the following devices have been heretofore proposed. A first type device is of the type which comprises an injection type electrode and a gate which opens in response to light in the photoelectric conversion portion thereof. As devices belonging to this first type device, for example, there has been proposed an image pickup device in which an amorphous semiconductor photoconductive layer having phototransistor characteristics and a read-out circuit are laminated one on the other, as disclosed in JP-A-61-222383, and also, there has been proposed a device utilizing a transistor of the electrostatic induction type as discussed in IEEE TRANSACTION ON ELECTRON DEVICES (vol. ED 22, 1975, pages 185-197). Either of these devices basically belongs to the injection type and the device utilizing amorphous semiconductor suffers from disadvantages such as a slow response, in particular. Further, in the transistor of the electrostatic induction type, an amplification portion is provided therein for every picture element, and therefore it is necessary to provide a complicated process so as to make amplification factors for the respective picture elements agree with each other. As a second type device, there has been proposed a system such as the one disclosed in JP-A-57-21876 in which p.sup.+ /.pi.p/n/n.sup.+ junctions are formed in amorphous silicon containing hydrogen in the same manner as in crystal silicon to thereby generate an avalanche action at the p/n junction so as to perform signal amplification. In this system, a good photo-response is obtained because the structure of this system is such that the charge injection from the electrode into the photoelectric conversion portion is blocked, while since the depletion layer thereof is not widened much, it suffers from a disadvantage that a satisfactory amplification factor is not always obtained.
As has been mentioned, the performance of presently available photoelectric conversion devices such as a photocell, a one-dimensional image sensor and a solid state imager of the photoconductive layer lamination type is not satisfactory and there has been a strong demand for an increased sensitivity and an improved performance.